The present invention relates to an optical assembly fabricated by mounting an optical device chip on a substrate, and in particular to an optical assembly of a passive alignment scheme suitable for reduction of the fabrication cost and a method for fabricating the same.
Reduction of fabrication cost of the optical device chip and the substrate is required of optical assemblies. In recent years, a passive alignment scheme (in which the optical device chip is not activated) is being studied vigorously instead of the conventional active alignment scheme (in which alignment is performed while the optical device chip is being activated).
Passive alignment schemes are divided broadly into two categories: index alignment scheme and self alignment scheme. In the index alignment scheme, alignment is performed by conducting image recognition on marks provided on the optical device chip and the substrate. In the self alignment scheme, alignment in the horizontal direction of the substrate is automatically performed by the surface tension of a solder bump which connects the optical device chip to the substrate. In order to reduce the dispersion of height in a direction perpendicular to the substrate, however, a measure such as control of the bump volume with high accuracy or formation of a standoff for positioning on the substrate is needed. In the existing circumstances, therefore, the index alignment scheme is considered to be more advantageous than the self alignment scheme.
As for the index alignment scheme, one described in Proceedings of 43rd Electronic Components and Technology Conference, pp. 808-817, 1993 and one described in 1994 Spring National Convention Record of the Institute of Electronics, Information, and Communication Engineers (IEICE), presentation No. C-291, for example, are known.
The optical assembly of the former-cited paper includes a laser chip of ridge waveguide type, a fiber carrier substrate, and an alumina substrate. Separately from them, an alignment plate is used. The chip, carrier substrate, and plate have an index having a cross-shaped ridge, an index having a cross-shaped hollow on a silicon nitride film, and an index having a cross-shaped opening in a chromic film, respectively. The chip and the carrier substrate are aligned by effecting image recognition on cross-shaped patterns while applying illumination transmitted through the plate. Thereafter, the chip and the carrier substrate are subjected to vacuum adsorption and soldered to the alumina substrate.
The optical assembly described in the latter-cited paper includes a laser chip and a silicon substrate. The substrate has a V-shaped groove for placing a fiber thereon. The chip and the substrate have an index formed by patterning an electrode circularly and an index formed by removing the electrode circularly, respectively. The chip is aligned by performing image recognition on the circular pattern while applying illumination transmitted through the substrate and the chip. The chip is soldered onto the substrate.
An object of the present invention is to provide an optical assembly including an optical device chip and a substrate each provided with indexes so as to allow image detection with high accuracy and formation with high accuracy.
Another object of the present invention is to provide a method for fabricating an optical assembly including an optical device chip and a substrate each provided with indexes so as to allow image detection with high accuracy and formation with high accuracy.
As a matter of course, improvement of alignment accuracy of the optical device chip and the substrate is important for optical assemblies. Typically the alignment accuracy is required to be equal to or less than the spot size of a light emission portion or a light receiving portion of the optical device chip or equal to or less than the spot size of an optical waveguide formed or mounted on the substrate. To be concrete, accuracy equal to or less than 1 xcexcm, for example, is needed. In the index alignment scheme, high accuracy formation of indexes with respect to the optical axis of the chip or the substrate and high-accuracy image recognition of these indexes are especially primary subjects. For performing high-accuracy image recognition, it is necessary to form, on the chip or the substrate, indexes for providing high-contrast images.
In the index of the laser chip described in the aforementioned paper, a silicon nitride film and a gold reflecting film are deposited on the top of the cross-shaped ridge surface and the peripheral around the ridge surface in order to improve the contrast between the top surface and the peripheral surface. An optical interference effect is used by controlling the thickness of the silicon nitride film so as to attain a thickness of 110 nm and by illuminating with monochromatic light. In the index of a fiber carrier substrate, the silicon nitride film is controlled to have a thickness of 120 nm in order to obtain the contrast between the bottom face of a cross-shaped hollow on the silicon nitride film and the top face.
Thus in the case where the top face of the ridge and the bottom face of the hollow which are parallel to the surface of the chip and the substrate are used, it is not easy to obtain high contrast images because they are parallel to each other. Considering dispersion in film thickness and dispersion in illumination of microscope optical system at the time of mass production, it is conjectured that variation in contrast, i.e., variation in alignment accuracy is caused. In the technique of the former-cited paper, therefore, the consideration given to the fabrication yield of optical assemblies is not sufficient.
In the latter-cited paper, metal electrodes of the laser chip and the silicon substrate are used as indexes. Patterning of the index of the chip is performed separately from the photolithography process for forming a laser active layer. Patterning of the index of the substrate is performed separately from the photolithography process for forming a V-shaped fiber groove.
In the case where a metal pattern is used as the index, there is a great difference in transmissivity or reflectivity between the material forming the chip or the substrate and the metal and hence a high-contrast image is obtained easily. However, patterning of the active layer or the V-shaped groove and patterning of metal are conducted separately. Because of the alignment error of the photomask, therefore, misalignment of the index of the active layer or the V-shaped groove is caused. In the technique of the latter-cited paper, the consideration given to the alignment accuracy of optical assemblies is insufficient.
As heretofore described, any optical assembly using conventional techniques cannot satisfy requirements of both high accuracy and simplicity for index alignment. Thus the present invention aims at realizing indexes which can be formed with high accuracy in an optical assembly and allows easy detection of a high-contrast image.
First, the present invention provides an optical assembly including an optical device chip and a substrate and having index alignment means which makes possible a reconciliation of high-accuracy formation and high-accuracy image detection;
Secondly, the present invention provides more concretely an index shape associated with the alignment means and suited for high-accuracy formation;
Thirdly, the present invention provides a basic index shape suited for formation and image measurement;
Fourthly, the present invention provides an index shape captured as an image;
Fifthly, the present invention provides index forming means suited for the case where the chip or the substrate includes a crystal;
Sixthly, the present invention provides index forming means more suited for the chip or the substrate;
Seventhly, the present invention provides means for configuring indexes suited for high-accuracy image detection;
Eighthly, the present invention provides means for forming indexes with high accuracy;
Ninthly, the present invention provides index forming means suited for the case where the chip or the substrate has an optical waveguide device;
Tenthly, the present invention provides index forming means suited for the case where the chip or the substrate includes a semiconductor optical device;
Eleventhly, the present invention provides chip placement means contributing to improvement of alignment accuracy in a vertical direction to the chip or the substrate;
Twelfthly, the present invention provides optical fiber placing means suited for the case where the chip or the substrate includes a silicon crystal having plane orientation (100);
Thirteenthly, the present invention provides configuration means for fixing a light emission/light receiving device and an optical waveguide substrate with high accuracy by means of index alignment; and
Fourteenthly, the present invention provides an assembly process for fixing the light emission/light receiving device and the optical waveguide substrate with high accuracy by means of index alignment.
First, an optical assembly according to the present invention includes an alignment index having, on at least one of the surface of an optical device chip and on the surface of a substrate for mounting the chip thereon which is opposed to the surface of the optical device chip, a face sloped with respect to the surface;
Secondly, an optical assembly according to the present invention includes an index having a hole or a moat;
Thirdly, an optical assembly according to the present invention includes an index having a quadrangular pyramid as a basic shape;
Fourthly, an optical assembly according to the present invention has such an image of the slope face of the index projected onto, for example, the surface of the chip or the substrate as to take the shape of a picture frame;
Fifthly, an optical assembly according to the present invention includes, in the chip or the substrate including a crystal, an index having a crystal plane as the slope face;
Sixthly, an optical assembly according to the present invention includes, in the chip or the substrate including a crystal, an index having the (111) crystal plane as the slope face;
Seventhly, an optical assembly according to the present invention includes an index sloped with respect to the surface of the chip or the substrate with a slope value equal to or larger than a value represented by sinxe2x88x921(n2/n1) where n1 and n2 are refractive index values of materials forming the slope face and n1 greater than n2;
Eighthly, an optical assembly according to the present invention includes an index etched simultaneously with etching of the device portion formed on the chip or the substrate, or an index etched by using, as an etching mask, the same material as at least a part of the device portion;
Ninthly, an optical assembly according to the present invention includes, in the chip or the substrate having an optical waveguide device, an index etched simultaneously with etching for defining the core of the optical waveguide device or etched by using the core material as the mask;
Tenthly, an optical assembly according to the present invention includes, in the chip or the substrate having an optical waveguide device, an index etched simultaneously with etching for defining an active area or a waveguide area of the semiconductor optical device; Eleventhly, an optical assembly according to the present invention includes the chip placed on the substrate in a flip chip manner;
Twelfthly, an optical assembly according to the present invention includes, in the chip or the substrate including a crystal, an index having a (111) plane and a V-shaped groove for disposing and optical fiber;
Thirteenthly, an optical assembly according to the present invention is obtained by mutually positioning and fixing, by means of image measurement, a slope face index formed simultaneously with the light emission/light receiving portion of the optical device and a slope face index formed simultaneously with the optical waveguide portion or the optical waveguide device mounting portion of the substrate; and
Fourteenthly, an optical assembly fabrication method according to the present invention includes the steps of superposing the optical device having a slope face index on a substrate having a slope face index, observing the indexes on images by using transmission illumination or vertical illumination of a microscope to detect positions of the optical device and the substrate, correcting the position deviation by using a fine movement stage, for example, and fixing the optical device to the substrate.
In accordance with a first aspect of the present invention, an alignment index having a slope face is illuminated from a direction perpendicular to the surface of the optical device chip or the substrate and is subjected to image recognition. The transmissivity/reflectivity of the index depends upon the slope angle. Since an evident brightness difference is caused between the index and the surface of the chip or the substrate, an image having a favorable contrast is obtained. The slope face of the index can be formed with high accuracy in the same way as the device portions of the chip and substrate. The slope angle is determined to be a constant value depending upon the material characteristics of the chip and substrate and the method of etching or the like. As a result, dispersion of contrast and alignment accuracy is small. Unlike the conventional technique of performing image recognition on planes which are parallel to each other, therefore, it is not necessary to control the minute film thickness of the index or intentionally use the optical interference effect. Furthermore, unlike the conventional technique of using the metal pattern as the index, misalignment of photolithography with respect to the device portion of the chip and substrate is not caused.
In accordance with a second aspect of the present invention, the slope face of the index is formed as the side face of a hole or the side face of a moat of a V-shaped groove or a U-shaped groove by the etching process in the same way as the device portion of the chip or the substrate. As compared with the case where the slope face is formed as the side face of a projection by the deposition process, the slope face can be easily formed with high accuracy.
In accordance with a third aspect of the present invention, the slope face of the index is formed basically as a side face of a quadrangular pyramid. As for the photomask pattern used for photolithography, a quadrilateral made up of straight lines is easier to make than a circle made up of curved lines as in the conventional technique. Since the slope face of the quadrangular pyramid is detected as a quadrilateral image, image processing such as centroid coordinate measurement can be conducted more easily than the cross image of the conventional technique. In the case where the chip or the substrate includes a crystal, the slope face can be formed with high accuracy and stably when a quadrangular pyramid is formed by anisotropic etching as compared to when a cone or the like is formed by isotropic etching.
In accordance with a fourth aspect of the present invention, a hollow frame pattern is detected as an index image and image processing is conducted. As compared with the case where all pixels of circular or cross-shaped smear-out pattern are calculated as in the conventional technique, the number of pixels to be calculated is reduced and processing time is shortened in the case of the frame pattern.
In accordance with a fifth aspect of the present invention, the slope face of the index is formed by using dependence of the etching speed upon the crystal orientation, i.e., by using anisotropy. Since a crystal plane of a slow etching speed is left as a slope face, dependence upon the process condition is not significant and the control performance is good.
In accordance with a sixth aspect of the present invention, a crystal plane having plane orientation (111) is formed as the slope face of the index. In crystals frequently used in the chip or the substrate, the etching speed of the (111) plane is typically the slowest as compared with other crystal planes. Therefore, the index is formed with higher accuracy.
In accordance with a seventh aspect of the present invention, light illuminated from a direction nearly perpendicular to the surface of the chip or the substrate is totally reflected by the slope face of the index. Therefore, the index looks black as an image, resulting in a very high contrast value.
In accordance with an eighth aspect of the present invention, the index is etched by using the same photomask as the device portion located on the chip or the substrate uses or by using, as the etching mask, the same material as used for the device portion. Accordingly, the index is formed in self alignment with the device portion. Unlike the conventional technique, therefore, misalignment of the photomask does not occur.
In accordance with a ninth aspect of the present invention, the index is formed in self alignment with the core of an optical waveguide device such as a waveguide type semiconductor laser, a waveguide type photodector, or a dielectric waveguide.
In accordance with a tenth aspect of the present invention, the index is formed in self alignment with the active area or the waveguide area of a semiconductor optical device such as a laser diode, a photodiode, or an optical switch.
In accordance with an eleventh aspect of the present invention, the chip is subjected to flip chip bonding, with a function layer being oriented to the substrate side. In the case where the chip is an active device, it means the junction down state. Therefore, accuracy of height from the substrate surface to the function layer is improved. Because in the chip fabrication process the height from the chip surface of the function layer side to the function layer is controlled with higher accuracy than the height from the chip surface of the opposite side to the function layer. Unlike the conventional technique in which flip chip bonding is not performed, the present invention technique does not need two process steps, i.e., the step of temporarily adsorbing the function layer side of the chip, and the step of thereafter fixing the opposite side to the substrate.
In accordance with a twelfth aspect of the present invention, a V-shaped groove having a (111) plane for mounting an optical fiber thereon is formed together with the indexes, on the chip including a silicon crystal of plane orientation (100) or on the substrate. The silicon crystal is widely used as the base material of photodetectors and dielectric optical waveguides. In addition, the silicon crystal allows etching with a very large anisotropy value and has mechanical strength comparing favorably with steel. Therefore, the silicon crystal is suitable for V-shaped grooves for fiber. Furthermore, the V-shaped groove is etched by using the same mask as the indexes use. Therefore, misalignment does not result.
In accordance with a thirteenth aspect of the present invention, indexes of the optical device and the substrate are formed in self alignment with their respective optical axes, position detection is performed on the basis of these indexes, and the optical device is mounted on the substrate.
In accordance with a fourteenth aspect of the present invention, indexes of both the optical device and the substrate are observed as clear images in the same field of view. Therefore, relative position deviation is detected with high accuracy. After this position deviation has been corrected, the optical device is fixed to the substrate.
First, according to the present invention, indexes can be formed with high accuracy and simply and high-contrast images can be subjected to measurement processing by forming indexes each having a slope face on surfaces of the optical device chip and the substrate. Therefore, alignment accuracy of the optical assembly can be improved simply. In turn, the fabrication cost can be reduced.
Secondly, a hole or a moat is formed on the surface of each of the chip and the substrate. Thereby, the present invention brings about an effect that the slope face of each index can be formed easily as the side face of the hole or the moat.
Thirdly, each index has a quadrangular pyramid as the basic shape. Thereby, the present invention brings about an effect that the slope face of the index can be formed with high accuracy and stably as the side face of the quadrangular pyramid. In addition, the present invention brings about an effect of facilitating image measurement processing because the image of the index is quadrilateral.
Fourthly, the slope face of the index takes the shape of a picture frame. Hence the image of the index becomes hollow. Therefore, the present invention brings about an effect of shortening the image processing time and reducing the cost.
Fifthly, anisotropy of the crystal forming the chip or the substrate is used. Thereby, the present invention brings about an effect that the index including a sloped crystal plane can be worked with high controllability.
Sixthly, the (111) crystal plane is used as the slope face of the index. Thereby, the present invention brings about an effect that the index can be worked with higher controllability.
Seventhly, illumination light is totally reflected at the slope face of the index. Thereby, an image with a high contrast value and a very large signal-to-noise ratio is obtained. Thus the present invention brings about an effect that position detection can be conducted with high accuracy.
Eighthly, the indexes are formed in self alignment with device portions of the chip and the substrate. Thereby, the present invention brings about an effect of higher accuracy of the indexes.
Ninthly, the present invention brings about an effect that the indexes can be formed with high accuracy with respect to the core of the optical waveguide device formed in each of the chip and the substrate, by using a self-alignment technique.
Tenthly, the present invention brings about an effect that the index can be formed with high accuracy with respect to the active area and the waveguide area of the semiconductor optical device of each of the chip and the substrate by using a self-alignment technique.
Eleventhly, the chip is flip-chip bonded to the substrate. Thereby, the present invention brings about an effect of improving the alignment accuracy of height from the surface of the substrate to the function layer.
Twelfthly, indexes each having the (111) plane and the V-shaped groove for optical fiber are simultaneously formed on the chip and the substrate each including a silicon crystal. Thereby, the present invention brings about an effect that misalignment between the indexes and the V-shaped groove can be prevented.
Thirteenthly, the optical axis of the light emission/light receiving portion of the optical device coincides with high accuracy with the optical axis of the optical waveguide portion of the substrate or the optical waveguide device mounting portion. Thereby, the present invention brings about an effect that an optical assembly having an optical coupling loss reduced to the minimum can be realized.
Fourteenthly, the present invention brings about an effect that a process for positioning and fixing the optical device and the optical substrate with high accuracy can be implemented by a simple system using a microscope, an image processing device, and,a fine movement stage.